Apparatus and process for preparing sorbents for mercury control at the point of use

ABSTRACT

A system for treating a contaminated gas stream is provided that includes a comminution device  204  operable to effect size reduction of a plurality of sorbent particles and form a plurality of comminuted particles, a plurality of nozzles  224  distributed through the gas stream and operable to introduce the plurality of comminuted particles into the gas stream, and a particle removal device  104  operable to remove at least most of the introduced comminuted particles and form a treated gas stream. The comminution device is in direct fluid communication with the nozzles.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 60/460,478, filed Apr. 3, 2003, the entire disclosure ofwhich is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed generally to treatment of gases toremove controlled materials and specifically to the treatment of fluegases to remove mercury and other contaminants.

BACKGROUND OF THE INVENTION

Each year, the emission of air toxics from combustion sources is beingsubjected to increasingly strict regulations. These regulations includenot only existing regulations, such as the U.S. Clean Air Act as amendedin 1970, 1977, and 1990, the National Energy Act, and the NationalPrimary and Secondary Ambient Air Quality Standards, but also pendingregulations that will require the removal of certain air toxics fromutility plant flue gas. Title III of the Amendments governs air toxics.As used herein, an “air toxic” refers to the 189 chemicals listed in theClean Air Act. Air toxics are present in the flue gas of combustionsources and appear both as particulate metals such as nickel, arsenic,and chromium in flyash particles and as vapor phase metals such asmercury, selenium, halogens, and halides and organic vapors. Vapor phaseair toxics are commonly present in flue gas in trace concentrations ofparts per million or less and therefore can be difficult to remove tocomply with pertinent regulations.

Several systems have been developed to remove trace quantities of airtoxics from flue gas. The systems have had varying degrees of success.

In one system, activated carbons and carbons treated chemically toproduce sulfide or iodide compounds with mercury are injected into a gasstream ahead of a particulate collection device, such as a Fabric Filter(FF), ElectroStatic Precipitator (ESP), Spray Dry Absorber (SDA), andFlue Gas Desulfurization (FGD) device. The activated carbon is typicallyintroduced into the gas stream by blowing the carbon, in a dryparticulate form, into the gas stream. While in flight, the carbonreacts with the entrained air toxics and binds the air toxics to thesurfaces of the carbon. The air toxic-containing carbon is then removedby the FF, ESP, SDA, and/or FGD.

The carbons used for the mercury removal process are generally of highsurface area and ground by the manufacturer to a small particle size,typically in the range of 10 microns to 100 millimeters. Manufacturersmake different grades of carbon depending on the specific propertiesdesired for a particular application. Activated carbon is manufacturedin a finished form at centralized manufacturing locations. The materialis then shipped in bulk form over long distances to end user locationsusing various means, such as supersacks, rail car and truck.

SUMMARY OF THE INVENTION

These and other needs are addressed by the various embodiments andconfigurations of the present invention. The present invention isdirected to the comminution of a sorbent before introduction into acontaminated gas stream. As used herein, a “sorbent” refers to asubstance having a capacity to adsorb, absorb, and/or otherwise entrap adesired material, such as an air toxic.

In one embodiment, the present invention is directed to a method fortreating a contaminated gas stream comprising one or more contaminantsthat includes the steps of:

(a) combusting a feed material, to produce a contaminated gas stream;

(b) comminuting a plurality of sorbent particles having a first sizedistribution to form comminuted sorbent particles having a second sizedistribution, the second size distribution being smaller than the firstsize distribution; and

(c) thereafter introducing the comminuted sorbent particles into the gasstream to remove the one or more contaminants.

The comminution of the sorbent particles is typically performed on siteand without intermediate storage. As used herein, “on-site” refers tothe general area of the combustion chamber, utility or other type ofplant and can include transporting continuously the sorbent particlesfrom a location near the chamber, utility or plant by means of aconveyor, slurry line, or pneumatic line, even though the location isnot on the physical site of the chamber, utility or plant. The timebetween the completion of the comminuting and introducing steps ispreferably no more than about 1 hour, no more than about 30 minutes, nomore than about 10 minutes, no more than about 1 minute, preferably nomore than about 30 seconds, and most preferably no more than about 1 to10 seconds. Normally, the comminution device is at least substantiallyco-located with the nozzles and remainder of the sorbent injectionsystem.

While not wishing to be bound by any theory, it is believed that thesometimes low contaminant removal rates of sorbents in commercialapplications are due in large part to the agglomeration of particlesduring transportation from the manufacturer to the end user and duringon-site storage before use. Agglomeration can dramatically change theeffective particle size distribution. In other words, agglomeration cancause a larger mean and median sorbent particle size. When the sorbentis introduced into the gas stream, the larger particle size distributionprovides, per unit of sorbent, fewer particles in the gas stream forcontaminant sorbtion and removal. This can cause a substantially reducedsorbent removal efficiency. To attempt to overcome this problem, endusers can introduce more sorbent per unit volume of gas stream, therebyresulting in much higher operating costs.

The method can have a number of benefits compared to existing processes.First, on-site milling can provide a sorbent particle size distributionthat is relatively small and highly effective in removing contaminants.As will be appreciated, a finer particle size provides a greater numberof particles in the gas stream for a given amount of sorbent, a highmass transfer rate to the gas stream, and a smaller interparticledistance in the gas stream. These factors can provide a higher rate ofcontaminant removal. On-site milling can also expose fresh, unreacted(more active) surfaces for contaminant capture. As will be appreciated,certain sorbents can react with the surrounding atmosphere when exposedto the atmosphere for prolonged periods, such as during shipping,thereby reducing the air toxic capacity of the sorbent. Second, the useof on-site milling permits the sorbent to be shipped as largerparticles, thereby providing higher packing densities during shipment,greater ease of material handling, less product loss duringtransportation and handling, and lower transportation and operatingcosts. For example, raw (unmilled) activated carbon can be produced atcentralized facilities, shipped to the end user site, and thereprocessed (milled) to produce a desired particle size. As will beappreciated, the bulk density of the finished (milled) activated productis much less than the bulk density of raw activated carbon. Third, thesorbent processed on site can meet tighter specifications, particularlysize distribution specifications. The on-site processed sorbentparticles can, for example, have a narrower and smaller sizedistribution than milled sorbent particles shipped long distances.Fourth, on-site processing permits additional ingredients (such as othersorbents) to be introduced into the mill along with the sorbent toproduce an enhanced sorbent material. The sorbent material, forinstance, can be a customized blend having desired properties that isuniquely suited for the specific application.

These and other advantages will be apparent from the disclosure of theinvention(s) contained herein.

The above-described embodiments and configurations are neither completenor exhaustive. As will be appreciated, other embodiments of theinvention are possible utilizing, alone or in combination, one or moreof the features set forth above or described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flue gas processing system according toan embodiment of the present invention;

FIG. 2 is a diagram depicting a portion of the sorbent injection systemaccording to an embodiment of the present invention;

FIG. 3 is a flow chart of the operation of the gas processing system ofFIG. 1; and

FIG. 4 is a graph showing mercury control using sorbents of 6micrometers mass mean particle diameter compared to sorbents of 18micrometers mass mean particle diameter.

DETAILED DESCRIPTION

A first embodiment of the flue gas processing system of the presentinvention is shown in FIG. 1. The system 100 includes a sorbentinjection system 112, particle removal device 104, and further treatmentdevice(s) 116, all positioned downstream of boiler 108. The treated gas120 is discharged through a stack into the exterior environment. As willbe appreciated, the boiler 108 combusts a suitable fuel, typically coal.The temperature of the flue gas 144 outputted from the boiler 108typically at least about 1,000 degrees F. When a heat exchanger (notpositioned) is positioned downstream of the boiler 108 and upstream ofthe injection system 112, the temperature of the flue gas 144 outputtedby the heat exchanger is in the range of about 250 to about 650 degreesF. (which is above the acid dew point).

The flue gas 144 includes a number of controlled contaminants. Forexample, a typical flue gas 144 includes from about 100–4,000 ppm sulfuroxides, from about 50–2,000 ppm nitrous oxides, and from about 1–200 ppmhydrogen chloride; from about 1–20 grains per actual cubic feet ofparticulates (at least most of which are flyash); and from about 0.1 ppbto about 10 ppm vapor phase air toxics. These amounts of thesecontaminants must be reduced to acceptable levels before emission of theflue gas into the environment.

The particulate removal device 104 is any suitable device for effectingremoval of at least most of the rejected sorbent particles. Typically,the device is a fabric filtration device such as a baghouse, a (wet ordry) electrostatic precipitator, a scrubber, a flue gas desulfurizer,and/or a combination thereof. The device 104 typically removes at leastabout 95% and more typically at least about 99% of the entrainedparticles from the flue gas 144.

The further treatment device(s) 116 can remove contaminants not removedsufficiently by the sorbent. Typically, the device(s) include a flue gasdesulfurizer and is preferably a wet flue gas desulfurizer. The flue gasdesulfurizer 116 typically removes at least most of the acid gases(e.g., sulfur dioxide and hydrogen chloride) and some of the air toxics.Examples of desulfurizers include spray towers, limestone, magnesiumlimestone, ammonia, forced oxidation, bubbling bed reactors, wetscrubbers, semi-wet scrubbers, dry scrubbers, and combinations thereof.The preferred desulfurizer is a wet scrubber having a vertical towerconstruction. The specific scrubbing agent(s) in the absorption zonedepend on the contaminants being removed. Examples of scrubbing agentsinclude an alkaline solution, such as a calcium-based slurry (such as asolution containing lime) and a sodium-based solution, for removingsulfur oxides (i.e., SO_(x), where x is 2, 3, or 4) and an ammonia-basedsolution, metals such as iodine dissolved in an organic solvent, organicsolvents such as methanol, and a solution of Fe(II) ions and thiosulfatein a miscible mixture of water and alcohol or a non-aqueous solvent, anemulsion of water-in-organic sulfoxides for removing both sulfur oxidesand/or nitrous oxides (i.e., NO_(x), where x is 1, 2, or 3). Typically,the flue gas desulfurizer 116 removes at least about 90% and moretypically at least about 98% of the SO₂; and at least about 80% and moretypically at least about 90% of the HCl present in the flue gas. Thedesulfurizer 116 typically reduces the temperature of the flue gas tothe bulk water dew point, which is commonly about 52.5° C.

As will be appreciated, a flyash particle removal system and flue gasdesulfurizer can be positioned upstream of the sorbent injection system112 as described in copending U.S. patent application Ser. No.10/804,654, filed Mar. 19, 2004, entitled “METHOD FOR REMOVING AIRBORNETOXIC MATERIALS FROM FLUE GAS” to Durham, et al., which is incorporatedherein by this reference. The flyash particles may be removed beforeinjection of the sorbent particles by the sorbent injection system 112or simultaneously with the removal of the sorbent particles by theparticle removal system 104.

When the FGD device precedes or is upstream of the injection system 112,the sorbent may be selected to remove contaminants in addition to airtoxics. The desulfurized flue gas output by the FGD commonly includesfrom about 5 to about 200 ppm sulfur oxides, and from about 0.2 to about40 ppm hydrogen chloride. In the desulfurizer, it is common to have someacid gas, such as sulfur trioxide and sulfuric acid, form an acid mistthat must be removed to produce a compliant waste gas. The acid mistforms when sulfur-containing gases are exposed in the desulfurizer witha scrubbing agent that is an alkaline medium to remove a majority of thesulfur species. A portion of the sulfur species forms the acid mist thatis typically not captured by the desulfurizer.

The sorbent injection system 112 further treats the gas to produce acompliant waste gas. The sorbent injection system 112 can have a numberof differing configurations depending on the sorbent employed. Thesorbent may be configured to remove air toxics, sulfur oxides, nitrousoxides, hydrochloric acid vapor, hydrogen sulfide vapor, acid gas,hydrogen fluoride, and/or condensibles (which include organiccompounds). The sorbent preferably is a free flowing particulate (carbonor non-carbon based) solid, such as activated carbon, molecular sieves,zeolites, chars, soots, aluminas, magnesium oxide, limestones, silicatessuch as the sorbents manufactured by Amended Silicates, LLC, mineralsorbents such as the sorbents manufactured by CDEM Holland B.Z., oranother suitable sorbent material and/or carbon, molecular sieves,zeolites, and other impregnable substrates including on exposed surfacesvarious substances, such as metals, metal compounds, sulfur, sulfurcompounds, and combinations and mixtures thereof.

Examples of suitable sorbents are discussed in U.S. Pat. Nos. 6,558,642;6,533,842; 6,524,371; 6,375,909; 6,372,187; 6,322,613; 6,270,679;6,136,281; 6,103,205; 5,900,042; 5,695,726; 5,670,122; 5,607,496;5,569,436; 5,460,643; 5,409,522; 5,322,128; 5,248,488; 5,245,106;5,225,175; 5,141,724; 5,120,515; 5,085,844; 5,064,626; 4,933,158;4,902,662; 4,892,567; 4,877,515; 4,843,102; 4,834,953; 4,814,152;4,786,484; 4,786,483; 4,771,030; 4,764,219; 4,721,582; 4,709,118;4,500,327; 4,474,896; 4,459,370; 4,369,167; 4,196,173; 4,101,631;4,094,777; and 3,755,161, each of which is incorporated herein by thisreference.

A preferred air toxic absorbent is activated carbon. Activated carbon isa mature and widely used technique for removing air toxics, such asmercury, from gas streams. Activated carbon is finely divided whichallows it to be introduced into a flowing gas stream and flow along withthe gas stream. Activated carbon, when entrained in the gas stream, hasbeen proven to collect air toxics through in-flight capture.

As shown in FIG. 2, the injection system 112 typically includes a hopper200 in fluid communication with a comminution device 204 that comminutesthe sorbent 208 to a desired size range. The bulk sorbent 208 istypically delivered by a supersack, pneumatic rail car or pneumatictruck. The comminuted sorbent 300 is conveyed pneumatically via an aireductor (not shown) powered by an air line 216 supplying air from theair source 218 (or air compressor). As air passes through the air line216 and past the eductor, solid particles outputted by the comminutiondevice 204 are aspirated and conveyed (by dilute phase techniques) viaair line 220 directly to a plurality of injection nozzles 224 a–f anddistributed substantially uniformly throughout the duct 228 as aplurality of solid particles.

The injection nozzles 224 can be in any suitable configuration, such asin a horizontal and/or vertical arrangement. Each nozzle typicallyincludes a pipe header of approximately 2 to 6 inches in diameter havinga plurality of about ¼ to 1 inch nozzles for uniform particulateinjection into the duct. The introduction rate of the sorbent isadjusted so that the gas downstream of the nozzles and upstream of theparticle removal system 104 includes from about 0.5 to 20 pounds ofsorbent per million actual cubic feet of gas. A preferred introductionrate is from about 3 to about 5 pounds of sorbent per million actualcubic feet of gas.

The size distribution of the comminuted sorbent 300 depends on theapplication. On the one hand, an extremely fine particle sizedistribution is desired to provide more particles for introduction intothe gas stream, thereby providing a greater degree of contaminantremoval for a given amount of sorbent. On the other hand, if theparticle size distribution is too fine the sorbent particle collectionefficiency of the particle removal system 104 will decrease, potentiallycausing an unsightly plume when the gas 120 is discharged from thestack. The comminuted sorbent particles preferably have a P₉₀ sizeranging from about 0.5 to about 25 microns, more preferably from about 1to about 10 microns, and even more preferably from about 0.2 to about 5microns. Typically, no more than about 20% of the particles have a sizegreater than about 100 microns and even more typically a size greaterthan about 10 microns. Immediately before being inputted into thecomminution device 204, the sorbent particles preferably have a P₉₀ sizeranging from about 10 microns to about 1 millimeter and no more thanabout 20% of the particles have a size greater than about 2 millimeters.The pre-comminuted sorbent can be in pelletized, granular, or powderedform.

To realize these fine particle sizes, and not intending to be limited bythe particular device, the preferred comminution devices are a jet millor agitated media attrition mill. As will be appreciated, a jet milluses a high pressure fluid stream, such as air, an inert gas such ashydrogen, or a liquid such as water, to effect particle comminution orsize reduction. The high pressure fluid is supplied to a grindingchamber via nozzles. High velocity (e.g., sonic or supersonic velocity)fluid (liquid or gas) exiting the nozzles accelerates the particlesintroduced into the jet mill resulting in size reduction due toparticle-to-particle collisions. The expanding fluid conveys thematerial to a particle size separator, such as a classifier, venturi,centrifuge, and the like, that rejects oversize material back to thegrinding zone and allows a predetermined particle size to pass through.The particle size distribution can be adjusted by changing thesetting(s) of the particle size separator, the air/liquid pressure,fluid flow, or nozzle size and configuration. The particles can beintroduced into the jet mill by any suitable technique, such as in afluidized bed and entrainment in the high pressure gas stream itself.Any type of jet mill may be employed such as the pancake jet mill, theloop jet mill, the opposing-nozzle jet mill, the Majac mill, the Gem-Tmill, the flooded grinding chamber mill, and the aeroplex or fluidizedbed jet mill. Examples of suitable jet mills include the JET-O-MIZER™,ROTO-SIZER™, and ROTO-JET™ by Fluid Energy Processing and EquipmentCompany, the NETZSCH-CONDUX™ jet mill by Netzsch, the Atritor DIRECTOPPOSED JET MILL™, the Kurimoto CROSS JET MILL™, ALPINE 50 AS™ and100AFG™ spiral jet mill, QYF™ fluidized-bed pneumatic jet mill, QYN™ultrasonic pneumatic jet mill, SINGLE TRACK™ jet mill, SK JET-O-MILL™CO-JET™ system, and the A-O™ jet mill. Examples of the agitated mediaattrition mills include, for example, Eirich Machines MAXXMILL™, and theUnion Process Inc. ATTRITOR™. As will be appreciated, sensors, such astemperature, pressure and/or sound sensors, may be located within themill and at the discharge of the mill to measure different processingparameters to measure the size of the sorbent particles created. Theinformation received from the sensors can be used to control the milloperation to produce the desired particle size. The use of such sensorsis further discussed in U.S. Pat. No. 6,318,649, which is incorporatedherein by this reference.

For optimal results and to avoid reagglomeration, the comminuted sorbentparticles are not commonly stored after comminution but introduceddirectly into the gas stream. Typically, the time interval betweenoutput from the comminution device to introduction into the gas streamis no more than about 1 hour, no more than about 30 minutes, no morethan about 10 minutes, no more than about 1 minute, preferably no morethan about 30 seconds, and most preferably no more than about 1 to 10seconds.

The operation of the system 100 will now be discussed with reference toFIG. 3.

In step 304, the sorbent particles are shipped, such as by pneumaticrailcar or truck, to the end user from the manufacturer and stored inthe hopper 200 by means of air line 230.

In step 308, the stored sorbent particles are transported via a device212 to the comminution device 204. The device 212 can be any suitablefeeding device, such as a rotary screw-type feeder, a conduit usinggravity to induce sorbent displacement, pneumatic feed line usingpressurized air and an eductor, and the like.

In step 312, the comminution device 204 effects size reduction of thesorbent particles, such as by high velocity gas flow, to realize thedesired size range. The size reduction is typically caused by highvelocity particle-to-particle contact and/or particle-to-stationarysurface contact. The typical size reduction factor of the sorbentparticles in this step is at least about 5 and more typically rangesfrom about 10 to about 200.

In step 316, the comminuted sorbent 300 is aspirated continuously into apressurized gas injection stream and transported by the air line 220 tothe injection nozzles spaced at intervals throughout the cross sectionof the contaminated gas stream. As noted, it is preferred that theoutputted comminuted sorbent 300 be fed continuously to the air line 220without intermediate storage to avoid particle reagglomeration.

In step 320, the nozzles inject the comminuted sorbent particles 300substantially uniformly throughout the gas stream. The contaminants arecaptured by the sorbent particles while the particles are in flight andby the sorbent particles that are captured in the particle removaldevice.

Entrained air toxics and other contaminants are captured effectively bythe entrained sorbent particles. Typically, at least about 50%, and moretypically at least about 75% of the desired contaminants, typically airtoxics, are captured by the sorbent particles. As an example ofcollecting mercury (an air toxic), mercury in the gas 144 leaving theboiler 108 at a concentration of 1–20 μg/m³ can be collected at anefficiency of from 10–99% depending on the amount and capacity of thesorbent being injected by the injection system 112. In this case theoutput waste gas 120 to the stack comprises no more than about 0.01–18μg/m³ of mercury and typically no more than about 0.05–10 μg/m³ ofmercury.

In step 324, the contaminant-laden particles are collected by theparticle removal system 104 and removed from the gas stream. Thecollected sorbent particles are removed periodically or continuouslyfrom the collection surface of the particle removal system 104 by anysuitable technique, such as liquid transfer, rapping, reverse airflowand the like.

EXPERIMENTAL Example 1

The following Example describes improved mercury control in flue gasachieved with finely ground sorbent particles.

This Example was carried out on the flue gas from an operatingcoal-fired power plant. A number of carbon sorbents, including DARCO FGDmanufactured by Norit Americas (Marshall, Tex.) with a mass meanparticle diameter of approximately 18 micrometers, was compared to DARCOINSUL, also manufactured by Norit Americas, with a mass mean particlediameter of approximately 6 micrometers, was injected into the flue gas,downstream of (i.e., after) an electrostatic precipitator (ESP), andupstream of a baghouse (fabric filter.) Flue gas temperature wasapproximately 300 degrees F. Mercury concentration was measured by aSemi-Continuous Emissions Monitor (S-CEM), in the following locations:upstream of the ESP and downstream of both the sorbent injection siteand the baghouse.

FIG. 4 shows the results. The X-axis depicts the amount of carboninjected into the flue gas per unit volume of flue gas (lbs carbon permillion actual cubic feet of flue gas treated, lbs/MMacf). The Y axisshows the measured mercury removal rate, measured as a percentageremoved between the upstream sampling location and downstream samplinglocation. The percentage removal was measured for each type of sorbent.Mercury removal results were obtained for one injection concentration ofDARCO INSUL only. The results are shown by the character referenced as“Insul” in the legend of FIG. 4. Mercury removal results were obtainedfor three injection concentrations of DARCO FGD. The results are shownby the characters referenced as “FGD, FGD2 and FGD3 in the legend ofFIG. 4. The results indicated that DARCO INSUL, the 6 micrometer massmean particle diameter sorbent, was more effective than the 18micrometer mass mean particle diameter sorbent to remove mercury fromflue gas streams.

A number of variations and modifications of the invention can be used.It would be possible to provide for some features of the inventionwithout providing others.

The present invention, in various embodiments, includes components,methods, processes, systems and/or apparatus substantially as depictedand described herein, including various embodiments, subcombinations,and subsets thereof. Those of skill in the art will understand how tomake and use the present invention after understanding the presentdisclosure. The present invention, in various embodiments, includesproviding devices and processes in the absence of items not depictedand/or described herein or in various embodiments hereof, including inthe absence of such items as may have been used in previous devices orprocesses, e.g. for improving performance, achieving ease and\orreducing cost of implementation.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of theinvention are grouped together in one or more embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description, with eachclaim standing on its own as a separate preferred embodiment of theinvention.

Moreover though the description of the invention has includeddescription of one or more embodiments and certain variations andmodifications, other variations and modifications are within the scopeof the invention, e.g. as may be within the skill and knowledge of thosein the art, after understanding the present disclosure. It is intendedto obtain rights which include alternative embodiments to the extentpermitted, including alternate, interchangeable and/or equivalentstructures, functions, ranges or steps to those claimed, whether or notsuch alternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

1. A method for treating a contaminated gas stream, the gas streamcomprising one or more contaminants, comprising: transporting aparticulate unmilled sorbent material and coal from a remote location toa utility plant site; combusting coal to produce a contaminated gasstream; milling the unmilled particulate sorbent material to produce aplurality of milled sorbent particles, the unmilled particulate sorbentmaterial having a first size distribution and the milled sorbentparticles having a second size distribution, wherein the second sizedistribution is smaller than the first size distribution, wherein thesecond size distribution has a P₉₀ size ranging from about 0.5 to about25 microns, and wherein the comminuting step occurs on-site with theplant in which the coal is combusted; and thereafter introducing thecomminuted sorbent particles into the gas stream to remove the one ormore contaminants.
 2. The method of claim 1, wherein the milled sorbentparticles are introduced directly into the gas stream after the millingstep.
 3. The method of claim 1, wherein the milling step is performed bya jet mill.
 4. The method of claim 1, wherein the milled sorbentparticles are flee of intermediate storage after the milling step. 5.The method of claim 1, wherein the one or more contaminants comprise anair toxic and wherein the time between the completion of the millingstep to the introducing step is no more than about 30 seconds.
 6. Themethod of claim 5, wherein sorbent material is activated carbon andwherein the air toxic is at least one of nickel, arsenic, chromium,mercury, selenium, lead, a halogen, and a halide.
 7. The method of claim1, wherein the first size distribution has a P₉₀ size ranging from about10 microns to about 1 millimeter.
 8. The method of claim 1, wherein, inthe milling step, the size reduction factor ranges from about 5 to about200.
 9. The method of claim 1, wherein the milling step comprises:entraining the particulate unmilled sorbent in a high velocity gasstream; and impacting the particulate unmilled sorbent at the velocityof the gas stream against at least one of another particle and astationary comminution surface to effect size reduction.
 10. A methodfor treating a contaminated gas stream, the gas stream comprising one ormore contaminants, comprising: transporting a particulate unmilledsorbent material from a remote location to a utility plant site;combusting coal to produce a contaminated gas stream; milling theunmilled particulate sorbent material to produce a plurality of milledsorbent particles, the unmilled particulate sorbent material having afirst size distribution and the milled sorbent particles having a secondsize distribution, wherein the second size distribution is smaller thanthe first size distribution, wherein the second size distribution has aP₉₀ size ranging from about 0.5 to about 25 microns, and wherein thecomminuting step occurs on-site with the plant in which the coal iscombusted; and introducing the comminuted sorbent particles into the gasstream to remove the one or more contaminants.
 11. The method of claim10, wherein the milled sorbent particles are introduced directly intothe gas stream after the milling step.
 12. The method of claim 10,wherein the milling step is performed by a jet mill.
 13. The method ofclaim 10, wherein the milled sorbent particles are free of intermediatestorage after the milling step.
 14. The method of claim 10, wherein theone or more contaminants comprise an air toxic and wherein the timebetween the completion of the milling step to the introducing step is nomore than about 30 seconds.
 15. The method of claim 14, wherein sorbentmaterial is activated carbon and wherein the air toxic is at least oneof nickel, arsenic, chromium, mercury, selenium, lead, a halogen, and ahalide.
 16. The method of claim 10, wherein the first size distributionhas a P₉₀ size ranging from about 10 microns to about 1 millimeter. 17.The method of claim 10, wherein, in the milling step, the size reductionfactor ranges from about 5 to about
 200. 18. The method of claim 10,wherein the milling step comprises: entraining the particulate unmilledsorbent in a high velocity gas stream; and impacting the particulateunmilled sorbent at the velocity of the gas stream against at least oneof another particle and a stationary comminution surface to effect sizereduction.
 19. A method for treating a contaminated gas stream, the gasstream comprising one or more contaminants, comprising: transporting aparticulate unmilled sorbent material from a remote location to autility plant site; providing a contaminated gas stream from combustionof coal; milling the unmilled particulate sorbent material to produce aplurality of milled sorbent particles, the unmilled particulate sorbentmaterial having a first size distribution and the milled sorbentparticles having a second size distribution, wherein the second sizedistribution is smaller than the first size distribution, wherein thesecond size distribution has a P₉₀ size ranging from about 0.5 to about25 microns, and wherein the comminuting step occurs on-site with theplant in which the coal is combusted; and introducing the comminutedsorbent particles into the gas stream to remove the one or morecontaminants.
 20. The method of claim 19, wherein the milled sorbentparticles are introduced directly into the gas stream after the millingstep.
 21. The method of claim 19, wherein the milling step is performedby a jet mill.
 22. The method of claim 19, wherein the milled sorbentparticles are free of intermediate storage after the milling step. 23.The method of claim 19, wherein the one or more contaminants comprise anair toxic and wherein the time between the completion of the millingstep to the introducing step is no more than about 30 seconds.
 24. Themethod of claim 19, wherein sorbent material is activated carbon andwherein the air toxic is at least one of nickel, arsenic, chromium,mercury, selenium, lead, a halogen, and a halide.
 25. The method ofclaim 19, wherein the first size distribution has a P₉₀ size rangingfrom about 10 microns to about 1 millimeter.
 26. The method of claim 19,wherein, in the milling step, the size reduction factor ranges fromabout 5 to about
 200. 27. The method of claim 19, wherein the millingstep comprises: entraining the particulate unmilled sorbent in a highvelocity gas stream; and impacting the particulate unmilled sorbent atthe velocity of the gas stream against at least one of another particleand a stationary comminution surface to effect size reduction.